Fluoride phosphor and light emitting device using the same and method of manufacturing the fluoride phosphor

ABSTRACT

A fluoride phosphor activated with tetravalent Mn can absorb blue light and emit red light, and is represented by the general formula: K 2 [M 1-a Mn 4+   a F 6 ] (M is at least one selected from group-IV elements of Ti, Zr, and Hf and group IVB elements of Si, Ge, Sn, and a is 0&lt;a&lt;0.2). The particles of the phosphor have a surface region where the concentration of tetravalent Mn is lower than the inner region of the phosphor particles.

BACKGROUND OF THE INVENTION

1. Technical Field

The present disclosure relates to a fluoride phosphor and a lightemitting device using the same, and a method of manufacturing thefluoride phosphor.

2. Background Art

A light emitting diode (LED) is a light emitting device which isfrequently used as a substitute for conventional light sources such asan incandescent bulb. Such a light emitting diode is useful as abacklight of display device, a warning lamp, an indicating lamp, and forluminarie. Also various light emitting devices which employ a laserdiode (LD) are proposed, in those, in a similar manner as in such alight emitting diode, a laser diode is used in combination with aphosphor. The light emitting diodes and laser diodes are bothsemiconductor light emitting elements fabricated using an alloy of groupIII-V compound such as gallium nitride (GaN). Combining such asemiconductor light emitting element and a phosphor, various lightemitting devices to emit light of a white color, an incandescent lampcolor, or an orange color have been developed. Those light emittingdevices to emit a white light etc., can be obtained based on theprinciple of light-color mixing. As for the system to emit a whitelight, there are well-known systems such as a system which employs anultraviolet-light emitting element and three types of phosphors whichemit lights of RGB colors respectively, and a system which employs ablue-light emitting element and a yellow-light emitting phosphor. Lightemitting devices of the type which employ a blue-light emitting elementand a yellow phosphor etc., are in demand in a wide range of fields suchas luminaries, e.g., a fluorescent-type lamp, an on-vehicle luminaire, adisplay, a backlight for liquid crystal. Of those, for the phosphorsused for a backlight of liquid crystal, in order to reproduce a widerange of colors on a chromaticity diagram, high color purity is alsodemanded along with the light emitting efficiency. Particularly, thephosphors used for backlights for liquid crystal are required to havecompatibility in combination with a filter, and a phosphor with a narrowhalf width of the emission peak has been in demand.

For example, as for red phosphors which have an excitation band inblue-color region and a narrow half width of the emission peak, fluoridephosphors having compositions such as K₂TiF₆:Mn⁴⁺, Ba₂TiF₆:Mn⁴⁺,Na₂TiF₆:Mn⁴⁺, and K₃ZrF₇:Mn⁴⁺ etc., are known (for example, see JP2009-528429A).

As for another phosphor, fluoride phosphor K₂SiF₆:Mn⁴⁺ is also known(for example, see JP 2010-209311A).

As for a yet another phosphor, excitation/emission spectra and mechanismof emission of a fluoride complex phosphor activated with Mn⁴⁺ is alsoknown (for example, see A. G. Paulusz, Effective Mn (IV) Emission inFluoride Coordination; J. Electrochemical Soc., 120 N7, 1973, p942-947).

Such a fluoride phosphor which is activated with tetravalent Mn and canemit red light with a narrow half width of the emission peak, and alight emitting device using the same are assumed to be particularlysuitable for the use as a backlight in a liquid crystal device. Thus,practical use of those has been demanded. On the other hand,conventional fluoride phosphors activated with tetravalent Mn have poorresistance to water. That is, Mn⁴⁺ which are tetravalent Mn present onthe surface of the phosphor particles are exposed to the moisture in theair and oxidized to manganese dioxide (MnO₂) which has a dark color,which is thought to resulting in degradation of the luminance.Accordingly, satisfactory durability cannot be obtained in a long-termreliability test, so that the usage which requires reliability, forexample, for LCD backlight applications and in-vehicle applications, hasbeen regarded difficult to implement.

The present invention is devised to solve the problems described above.One object of certain embodiments of the invention is to obtain a waterresistant red light-emitting phosphor having a narrow half width of theemission peak, and a light emitting device using the same, and a methodof manufacturing the phosphor.

SUMMARY

A fluoride phosphor according to an aspect of the disclosure is afluoride phosphor activated with tetravalent Mn which can absorb bluelight and emit red light. The fluoride phosphor is represented by thegeneral formula shown below, and the particles of the phosphorrespectively have a surface region where the concentration oftetravalent Mn is lower than the inner region.

K₂[M_(1-a)Mn⁴⁺ _(a)F₆] (where M is at least one selected from group-IVelements of Ti, Zr, and Hf and group IVB elements of Si, Ge, Sn, and ais 0<a<0.2).

Further, according to another aspect of the invention, a method ofmanufacturing a fluoride phosphor which is a fluoride phosphor activatedwith tetravalent Mn to absorb blue light and emit red light, having ageneral formula K₂[M_(1-a)Mn⁴⁺ _(a)F₆] (where M is at least one selectedfrom group-IV elements of Ti, Zr, and Hf and group IVB elements of Si,Ge, Sn, and a is 0<a<0.2); the method may include forming phosphorcores, including mixing a solution which contains at least Mn and F, asolution which contains at least K and F, and a solution which containsat least Si and F, which are to be described below; and forming asurface region on each phosphor core, including mixing the phosphorcores and a solution containing a reducing agent to form a surfaceregion on the phosphor cores to form phosphor particles so that aconcentration of tetravalent Mn on the surface region is lower than inthe inner region of each phosphor particle.

According to the embodiments, a red light-emitting phosphor can beobtained in which good light emission characteristics and excellentwater resistance are realized. Also, a red light-emitting phosphor canbe utilized with narrower half width of the emission peak.

The method of manufacturing a phosphor according to the embodiments canprovide a red light-emitting phosphor with a narrow half width of theemission peak, in which good light emission characteristics andexcellent water resistance are realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view approximately showing theconcentration distribution of Mn⁴⁺ in a phosphor particle.

FIG. 2 is a schematic cross-sectional view of a light emitting deviceaccording to an embodiment of the present invention.

FIG. 3 is a schematic plan view showing a light emitting deviceaccording to an embodiment of the present invention.

FIG. 4 is a graph showing an emission spectrum of a fluoride phosphoraccording to Example 6.

DETAILED DESCRIPTION OF EMBODIMENTS

The embodiments according to the present invention will be describedbelow with reference to the drawings. The preferred embodiments areintended as illustrative of a phosphor and a light emitting device usingthe same, and a method of manufacturing the phosphor to give a concreteform to technical ideas of the present invention, and the scope of theinvention is not limited to those described below.

In the specification, the relation between the color names and thechromaticity coordinates, the relation between the range of wavelengthof light and the color name of single color light, and the like, conformto JIS Z8110. More specifically, 380 nm to 455 nm correspond to bluepurple light, 455 nm to 485 nm correspond to blue light, 485 nm to 495nm correspond to blue green light, 495 nm to 548 nm correspond to greenlight, 548 nm to 573 nm correspond to yellow green light, 584 nm to 610nm correspond to yellow red light, and 610 nm to 780 nm correspond tored light. In the specification, light in short-wavelength region ofvisible light is not specifically limited, but may be indicated to a 400nm to 500 nm region.

As explained above, a conventional fluoride phosphor activated withtetravalent Mn have a drawback of poor water resistance. The inventorsuspects the reason of such problem as Mn⁴⁺ which are tetravalent Mnpresent on the surface of the phosphor particles are exposed to thewater (moisture) in the air then oxidized to manganese dioxide (MnO₂)which shows a dark color, resulting in degradation of the luminance.

To solve this problem, a fluoride phosphor according to the embodimentof the present invention has been developed, which is represented by thegeneral formula shown below, and its particle forms a surface regionwhere the concentration of tetravalent Mn is lower than its innerregion.

In particular, a fluoride phosphor according to an aspect of thedisclosure is activated with tetravalent Mn and can absorb blue lightand emit red light. The phosphor is represented by the general formulashown below, and the phosphor particles respectively have a surfaceregion where the concentration of tetravalent Mn is lower than the innerregion.

K₂[M_(1-a)Mn⁴⁺ _(a)F₆] (where M is at least one selected from group-IVelements of Ti, Zr, and Hf and group IVB elements of Si, Ge, Sn, and ais 0<a<0.2).

The phosphor particles according to an embodiment are preferablyarranged so that the concentration of tetravalent Mn is approximatelyuniform in the inner region of the phosphor particles, and is graduallydecreased toward the surface in the surface region. With thisarrangement, while maintaining the characteristics of broad colorreproduction range with the main portion of the phosphor which is afluoride phosphor activated with tetravalent Mn, the existenceprobability of tetravalent Mn at the surfaces of the particles arereduced, so that even in the case of surface elution due to humidity orthe like, generation of MnO₂ can be suppressed which further suppressingblackening. Thus, degradation in the emission intensity can be reduced.

Further, the concentration of tetravalent Mn in the surface region ispreferably 30% or less with respect to the concentration of tetravalentMn in the inner region.

Further, the fluoride phosphor is preferably prepared so that an elutionamount of tetravalent Mn obtained by charging the fluoride phosphor in 1to 5 times amount of pure water relative to the amount of the phosphorto dissolve surface of the phosphor particles is in a range of 0.05 to 3ppm.

Moreover, the fluoride phosphor is preferably prepared so that aluminous flux maintaining rate obtained by charging the fluoridephosphor in 1 to 5 times amount of pure water relative to the amount ofthe phosphor is 90% or greater.

In the general formula described above, M is preferably Si or Si and Ge.

Further, a light emitting device which includes a light source to emit ashort-wavelength visible light and the fluoride phosphor to absorb thelight and emit a red light can be obtained. As described above,employing a fluoride phosphor activated with tetravalent Mn allow forobtaining of a light emitting device which has a wider colorreproduction range and excellent light emission characteristics thanlight emitting devices with conventional fluoride phosphors.

EMBODIMENTS

In reliability tests of fluoride phosphors under humid conditions,tetravalent Mn serves as an activator and are located on the surface ofthe phosphor particles react with water to create MnO₂ which has a darkcolor, resulting in degradation of the luminance. As a result ofintensive studies on the surface composition of fluoride phosphors, thepresent inventor found that forming a surface region with lowertetravalent Mn on the phosphor particles while maintaining the samecomposition as a phosphor allows for an improvement in the waterresistance of the phosphor.

The fluoride phosphor according to the present embodiment can berepresented by a general formula K₂[M_(1-a)Mn⁴⁺ _(a)F₆] (wherein M is atleast one selected from group-IV elements of Ti, Zr, and Hf and groupIVB elements of Si, Ge, Sn, and a is 0<a<0.2). The fluoride phosphor isactivated with tetravalent Mn. The particles of the fluoride phosphorhave a surface region having the same composition as that of the innerregion except for a smaller amount of tetravalent Mn. With thisarrangement, fluoride phosphors which can be excited by ashort-wavelength visible light to emit light of red region and hasexcellent water resistance can be realized. Also, a fluoride phosphorhaving an emission spectrum with a narrow half width of the emissionpeak can be realized.

Surface Region

The surface region described above includes a low tetravalent Mnconcentration region where the concentration of tetravalent Mn is lowerthan that in the inner region. The surface region is not demarcatedclearly by, for example, a two-layered structure, from the inner region,but as shown in FIG. 1 where the tetravalent Mn are shown by dots, thesurface region is formed on the phosphor particles 71 with graduallydecreasing concentration toward the surface. The change in theconcentration of tetravalent Mn is not limited to the state in which theconcentration is decreased rather constantly from the phosphor core thatis the inner region 73 in FIG. 1. But as shown in FIG. 1, theconcentration of tetravalent Mn is preferably approximately uniform inthe inner region 73 of the phosphor particles 71, while in the surfaceregion 72 which corresponds to the surface portion of the phosphorparticles 71, the concentration of tetravalent Mn decreases toward thesurface. With this arrangement, while maintaining the characteristics ofbroad color reproduction range compared to that of conventional fluoridephosphors, with the main portion of the phosphor which is a fluoridephosphor activated with tetravalent Mn, even in the case where thesurfaces of the phosphor particles 71 are eluded due to humidity, withthe lack of tetravalent Mn on the surface, the generation of MnO₂ can besuppressed which further suppressing blackening. Thus, degradation inthe emission intensity can be reduced. It should be noted that thesurface region 72 shown in FIG. 1 is in an ideal state. The surfaceregion 72 is not necessarily formed to completely cover the entire innerregion 73 of the phosphor particles 71, and such a state is alsoincluded in the present invention. That is, a state of the phosphorparticles 71 having a part of the inner region 73 exposed on the surfaceregion 72 to a degree that can maintain sufficient resistance tomoisture is also included in the present invention.

The concentration of tetravalent Mn in the surface region is preferably30 weight % or less with respect to the tetravalent Mn in the innerregion of the phosphor. Further, the concentration of tetravalent Mn inthe surface region is preferably 25 weight % or less, more preferably 20weight % or less, with respect to that in the inner region. Meanwhile,the concentration of tetravalent Mn in the surface region can be 0.5weight % or more with respect to that of the inner region. As describedabove, the concentration of tetravalent Mn is ideally approximatelyzero, but an increase in the surface region with a low tetravalent Mnincreases the region which does not contribute to emission on thesurface of the phosphor particles, which would lead a decrease of theemission intensity.

Although depending on the particle diameter of the phosphor, thethickness of the surface region is preferably about 1/20 to about 1/50with respect to the average particle diameter. For example, in the casewhere the phosphor particles have an average particle diameter of 30 to50 ìm and a median particle diameter of 40 to 60 ìm, the thickness ofthe surface region may be 1 ìm or less.

Further, the fluoride phosphor is preferably prepared so that an elutionamount of tetravalent Mn obtained by charging the fluoride phosphor in 1to 5 times amount of pure water relative to the amount of the phosphorto dissolve surface of the phosphor particles is in a range of 0.05 to 3ppm. The elution amount of tetravalent Mn is preferably in a range of0.1 to 2.5 ppm and more preferably in a range of 0.2 to 2.0 ppm. This isbecause, although the lower the elution amount of tetravalent Mn thehigher the water resistance will be, the greater the ratio of surfacearea with a low concentration of tetravalent Mn will result in a greaterdegradation of emission intensity.

Such an arrangement described above allows for suppressing thedegradation of emission luminance due to coloration caused by MnO₂attributable to Mn⁴⁺ eluded in the water, and thus a fluoride phosphorwith high water resistance can be realized. In the general formuladescribed above, M is preferably Si or Si and Ge. With this arrangement,a fluoride phosphor with high optical intensity can be provided.

Method of Manufacturing Fluoride Phosphor

(1) Mixing a solution which contains at least Mn and F, a solution whichcontains at least K and F, and a solution which contains at least Si andF to form a precipitate (phosphor)

In the specification, the term “a solution which contains at least Mnand F (hereinafter may be called as “a solution A”)” refers to asolution of hydrofluoric acid which contains a Mn source. Examples of Mnsource in the solution A include K₂MnF₆, KMnO₄, and K₂MnCl₆. Amongthose, K₂MnF₆ is preferable, because K₂MnF₆ does not contain Cl whichcauses distortions in crystal lattice which may result in instability ofthe crystal, so that while maintaining its oxidation number(tetravalent) which can serve as an activator, allowing for stablepresence in the hydrofluoric acid as MnF₆ complex ions. Of the Mnsources, a source which contains K also serves as a K source.

In the specification, the term “a solution which contains at least K andF (hereinafter may be called as “a solution B”)” refers to a solution ofhydrofluoric acid which contains a K source. Examples of K source in thesolution B include water-soluble potassium salts such as KF, KHF₂, KOH,KCl, KBr, Kl, potassium acetate (CH₃COOK), and K₂CO₃. Among those, KHF₂is preferable, because KHF₂ is soluble without decreasing theconcentration of hydrogen fluoride in the solution, and has a small heatof dissolution which contributes high safety.

Those Mn sources and K sources may be used singly or in combination oftwo or more kinds.

In the specification, the term “a solution which contains at least Siand F (hereinafter may be called as “a solution C”)” refers to anaqueous solution which contains a SiF₆ source. The SiF₆ source containedin the solution C is a compound which contains Si and F with goodsolubility to the solution. Examples thereof include H₂SiF₆, Na₂SiF₆,(NH₄)₂SiF₆, Rb₂SiF₆, and Cs₂SiF₆. Among those, H₂SiF₆ is preferable,because H₂SiF₆ has high solubility to water and does not contain analkali metal element as inevitable impurities. Those SiF₆ sources may beused singly or in combination of two or more kinds.

The concentration of hydrogen fluoride in the solution A is typically 30weight % or greater, preferably 35 weight % or greater, more preferably40 weight % or greater, and typically 70 weight % or less, preferably 65weight % or less, more preferably 60 weight % or less. In the case wherethe concentration of hydrogen fluoride is too low, K₂MnF₆ which is theraw material of activator element and contained in the solution A isunstable and easily hydrolyzed, which may cause abrupt change in the Mnconcentration. Thus, control of amount of Mn as the activator in thesynthesized phosphor becomes difficult, which tends to widen thevariation in luminous efficiency of the phosphor. On the other hand, inthe case where the concentration of hydrogen fluoride is too high, theboiling point decreases and generation of hydrogen fluoride gas tends totake place. Thus, control of concentration of hydrogen fluoride in thesolution becomes difficult, which tends to widen the variation in theparticle size of the phosphor.

The concentration of K source in the solution B is typically 20 weight %or greater, preferably 25 weight % or greater, more preferably 30 weight% or greater, and typically 70 weight % or less, preferably 65 weight %or less, more preferably 55 weight % or less. In the case where theconcentration of K is too low, the yield of the phosphor tends todecrease. In the case where the concentration of K is too high, theparticle size of the phosphor tends to be too small.

The mixing method of the solutions A to C is not specifically limited,and the mixing may be performed by adding the solutions B and C into thesolution A while stirring the solution A, or by adding the solutions Aand B into the solution C while stirring the solution C. Alternatively,all the solutions A to C may be charged at once and stirring and mixingis performed.

Mixing of the solutions A to C allows reaction of the Mn-source, theK-source, and the SiF₆-source to precipitate desired phosphor crystals.The crystals are collected by using a solid-liquid separation technique,such as filtering, and washed with a suitable solution such as ethanol,water, acetone. The filtered crystals are then tried at a temperaturegenerally 50° C., preferably 55° C. or greater, and generally 110° C. orless, preferably 100° C. or less, more preferably 90° C. or less. Thedrying time is not specifically limited as long as the moisture adheredto the phosphor can be evaporated, and for example, drying may beperformed for about 10 hours.

At the time of mixing of the solutions A to C, considering a differencebetween the charge composition of raw materials of the phosphor and thecomposition of the phosphor product, the mixing ratio of the solutions Ato C is needed to be adjusted so that the phosphor product has a desiredcomposition.

(2) Mixing the phosphor product (phosphor core) obtained above, asolution which contains at least Si and F (hereinafter may be referredto as a “solution D”), a solution which contains a reducing agent, and asolution which contains at least K and F (hereinafter may be referred toas a “solution E”, to form a surface region on the surfaces of phosphorparticles so that the surface region has a concentration of tetravalentMn lower than that of inner side of the phosphor particles.

In the specification, the term “a solution which contains at least Siand F (hereinafter may be called as “a solution D”)” refers to anaqueous solution which contains a SiF₆ source.

The SiF₆ source contained in the solution D is a compound which containsSi and F with good solubility to the solution. Examples thereof includeH₂SiF₆, Na₂SiF₆, (NH4)₂SiF₆, Rb₂SiF₆, and Cs₂SiF₆. Among those, H₂SiF₆is preferable, because H₂SiF₆ has high solubility to water and does notcontain an alkali metal element as inevitable impurities. Those SiF₆sources may be used singly or in combination of two or more kinds.

The concentration of hydrogen fluoride in the solution D is typically 25weight % or greater, preferably 30 weight % or greater, more preferably35 weight % or greater, and typically 65 weight % or less, preferably 60weight % or less, more preferably 55 weight % or less. The concentrationof SiF₆-source in the solution D is typically 3 weight % or greater,preferably 5 weight % or greater, and typically 40 weight % or less andpreferably 30 weight % or less.

For the solution which contains a reducing agent, a solution whichcontains hydrogen peroxide or oxalic acid can be used. Of those,hydrogen peroxide is preferable, because hydrogen peroxide allowsreduction of Mn without affecting the base particles of the phosphor andto be eventually decomposed into harmless oxygen and hydrogen, whichfacilitates the use in manufacturing and has a low environmental burden.

In the specification, the term “a solution which contains at least K andF (hereinafter may be called as “a solution E”)” refers to a solution ofhydrofluoric acid which contains a K-source. Examples of K-source in thesolution E include water-soluble potassium salts such as KF, KHF₂, KOH,KCl, KBr, Kl, potassium acetate (CH₃COOK), and K₂CO₃. Among those, KHF₂is preferable, because KHF₂ is soluble without decreasing theconcentration of hydrogen fluoride in the solution, and has a small heatof dissolution which contributes high safety.

Mixing of the solutions D and E may be carried out by stirring thesolution D which contains Si and F and to which a phosphor (phosphorcore) has been added, a solution which contains a reducing agent and thesolution E are added in this order.

The products are collected by using a solid-liquid separation technique,such as filtering, and washed with a suitable solution such as ethanol,water, acetone. The filtered crystals are then tried at a temperaturegenerally 50° C., preferably 55° C. or greater, more preferably 60° C.or greater, and generally 110° C. or less, preferably 90° C. or less.The drying time is not specifically limited as long as the moistureadhered to the phosphor can be evaporated, and for example, drying maybe performed for about 10 hours.

Light Emitting Device

Further, a light emitting device can be fabricated with the use of thefluoride phosphor in combination with a light source which can emit ashort-wavelength visible light. For example, a white light emittingdevice has been developed with employing a combination of a blue lightemitting LED and a phosphor which can be excited by the blue light toemit yellow light, but such a light emitting device tends to haveinsufficient red color component. Thus, with a combination of thefluoride phosphor described above to absorb the blue light from thelight source and add red fluorescent light, a light emitting device withhigher color rendering properties and high quality can be realized.Particularly, in the used for the backlights of liquid crystals, afluoride phosphor which has a narrow half width of the emission peak anda high emission intensity is preferably used. With this arrangement, alight emitting device to emit more vivid red color light can beprovided.

Next, such light emitting devices which employs a fluoride phosphor asdescribed above will be described. Examples of the light emittingdevices include luminaries such as fluorescent lamp type light-emittingdevices, indicating devices such as display devices and radar devices,and backlights of liquid crystal devices. The fluoride phosphorsaccording to the present embodiment are preferably used particularly indisplays. For the excitation light source of the light emitting devices,a light emitting element which can emit a light of a short-wavelengthregion of visible light can be used. With the light emitting devices inwhich the excitation light source is covered with a sealing resin whichcontains a phosphor, a part of light emitted from the excitation lightsource can be transmitted through the phosphor, and emitted to outside.Effective use of the light thus emitted to outside as a part of thedesired mixed-color light can significantly reduce the loss of lightemitted from the light emitting device, so that light emitting devicesof high efficiency can be provided.

The light emitting devices equipped with a light emitting device includevarious types such as a lamp-type and a surface-mounting type.Generally, a “lamp type” indicates a type which has an exterior surfacemade with a lamp-shape resin member. Also, a “surface-mounting type”generally indicates a type which has a recess-shaped housing portion inwhich a light emitting element is mounted and in which a resin isfilled. Further examples of other types of light emitting device includea type in which a light emitting element is mounted on a planar mountingsubstrate and a sealing resin which contains a phosphor is disposed in alens shape to cover the light emitting element.

One exemplary light emitting device which employs a fluoride phosphorwill be described below. FIG. 2 is a schematic plan view of a lightemitting device according to the present embodiment. FIG. 3 is aschematic plan view of a light emitting device according to the presentembodiment. It should be noted that FIG. 2 shows cross section takenalong line II-II in FIG. 3. The light emitting device is an example ofsurface-mounting type light emitting devices.

The light emitting device 100 includes a light emitting element 10 whichis a gallium nitride-based compound semiconductor and can emit ashort-wavelength visible light, and a molded body 40 where the lightemitting element 10 is mounted. The molded body 40 includes a first lead20 and a second lead 30 and is integrally molded with the use of athermoplastic resin or a thermosetting resin. The molded body includes arecessed portion defined by a bottom surface and side surfaces, and alight emitting element 10 is mounted on the bottom surface of therecess. The light emitting element 10 includes a pair of electrodeswhich are respectively electrically connected to the first lead 20 orthe second lead 30 via a wire 60. The light emitting element 10 issealed with the sealing member 50. For the sealing member 50, athermosetting resin such as an epoxy resin, a silicone resin, anepoxy-modified silicone resin, or a modified silicone resin ispreferably employed. The sealing member 50 contains a fluoride phosphor70 which can convert the wavelength of the light from the light emittingelement 10. The main components of the light emitting device100 will bedescribed below.

Light Emitting Element

A light emitting element which can emit a short-wavelength visible lightcan be used. Particularly, a range of 420 nm to 485 nm is preferable.More preferably, a light emitting element has an emission peakwavelength in a range of 440 nm to 480 nm. With this arrangement, afluoride phosphor can be excited efficiently which allows for effectiveuse of the visible light. With this arrangement, a fluoride phosphorwith high optical intensity can be provided. The use of such a lightemitting element for the excitation light source allows for high degreeof linearity between input and output, and a light emitting device whichis durable to mechanical shock can be obtained. The visible light of ashort wavelength region is mainly in the blue light-region.

Emission Spectrum

A fluoride phosphor absorbs a short-wavelength visible light and has anemission peak wavelength at longer wavelength side than the emissionpeak wavelength of the excitation light. It is preferable that thevisible light of a short wavelength region is mainly in the bluelight-region. More specifically, it is preferable that the phosphor isexcited with a light from an excitation light source which has anemission peak wavelength between 400 nm to 500 nm, and has an emissionpeak wavelength in a range of 610 nm to 650 nm, with a half value widthof the emission spectrum of 2 nm or greater and 10 nm or less. For theexcitation light source, a light source which has an emission peakwavelength mainly between 420 nm to 485 nm, and a light source which hasan emission peak wavelength between 440 nm to 480 nm is more preferablyused.

Other Phosphors

The fluoride phosphors according to the present embodiment can be usedsingly or in combination with other phosphors. The other phosphors mayabsorb light from the light emitting element and convert it to light ofdifferent wavelength. Examples of other phosphors preferably include atleast one selected from a nitride-based phosphor, an oxynitride-basedphosphor, and a sialon-based phosphor which are activated with alanthanoid element such as Eu and Ce, or the like, an alkaline earthhalogen apatite phosphor, an alkaline earth metal borate halogenphosphor, an alkaline earth metal aluminate phosphor, an alkaline earthsulfide phosphor, an alkaline earth thiogallate phosphor, an alkalineearth silicon nitride, and a germinate which are mainly activated with alanthanoid element such as Eu and/or a transition metal such as Mn, arare earth aluminate and a rare earth silicate which are mainlyactivated with a lanthanoid element such as Ce, or an organic compoundand an organic complex which are mainly activated with a lanthanoidelement such as Eu. Specific examples include (Ca,Sr,Ba)₂SiO4:Eu,(Y,Gd)₃(Ga,Al)₅O₁₂:Ce, (Si,Al)₆(O,N)₈:Eu(β-sialon), SrGa₂S₄:Eu,(Ca,Sr)₂Si₅Ns:Eu, CaAlSiN₃:Eu, and (Ca,Sr)AlSiN₃:Eu. Accordingly, lightemitting devices with various color tones can be provided.

Example 1

The fluoride phosphors according to Examples 1 to 9 and ComparisonExample 1 will be described below. Table 1 shows the charging amount ofthe raw materials of the fluoride phosphor according to ComparisonExample 1. Table 2 shows the charging amount of the raw materials of thefluoride phosphors according to Examples 1 to 9. Further, FIG. 4 showsthe emission spectrum of the fluoride phosphor according to Example 6.

TABLE 1 Solution C Solution B Charged Solution A Charged Charged AmountAmount (g) Amount (g) (g) 40% Charge Composition 55% HF 55% HF H₂SiF₆Ratio (mol) aqueous aqueous aqueous K Si Mn F K₂MnF₆ solution KHF₂solution solution COMPARISON 2 0.95 0.05 6 16.25 1000 195.10 200 450EXAMPLE

TABLE 2 Solution D Solution E Fluorescent Charge Charged ChargedMaterial Reducing Composition Ratio Amount (g) Amount (g) Core (g) Agent(g) (mol) 40% 55% 55% COMPARISON 30% KSF H₂SiF₆ HF HF EXAMPLE H₂O₂ Coreaqueous aqueous aqueous Fluorescent aqueous K Si (g) solution solutionKHF₂ solution Material solution EXAMPLE 1 0.25 0.25 1 12.27 150 2.66 6030.0 1.5 EXAMPLE 2 0.25 0.25 1 24.53 150 2.66 60 30.0 1.5 EXAMPLE 3 0.250.25 1 36.81 150 2.66 60 30.0 1.5 EXAMPLE 4 0.50 0.50 1 12.27 150 5.3260 30.0 1.5 EXAMPLE 5 0.50 0.50 1 24.53 150 5.32 60 30.0 1.5 EXAMPLE 60.50 0.50 1 36.81 150 5.32 60 30.0 1.5 EXAMPLE 7 1.00 0.75 1 12.27 15010.64 60 30.0 1.5 EXAMPLE 8 1.00 0.75 1 24.53 150 10.64 60 30.0 1.5EXAMPLE 9 1.00 0.75 1 36.81 150 10.64 60 30.0 1.5

Comparison Example 1

First, a method of manufacturing a fluoride phosphor according toComparison Example 1 which serves as a phosphor core will be described.In order to obtain the charge composition ratio shown in Table 1, 16.25g of K₂MnF₆ was weighed and dissolved in 1000 g of 55 wt % HF aqueoussolution to prepare the solution A. Meanwhile, the solution B wasprepared by weighing 195.10 g of KHF₂ and dissolving it in 200 g of 55wt % HF aqueous solution. Also, 450 g of 40 wt % H₂SiF₆ aqueous solutionwas weighed to prepare the solution C. Then, while stirring the solutionA, the solution B and the solution C were added simultaneously. Theprecipitated product was isolated and washed with IPA and dried at 70°C. for 10 hours. Thus, the fluoride phosphor according to ComparisonExample 1 which serves as a phosphor core was prepared.

Example 1

Next, processes of manufacturing the fluoride phosphors according toExample 1 will be described below. In order to obtain the chargecomposition ratio shown in Table 2, 12.27 g of 40 wt % H₂SiF₆ aqueoussolution was weighed and 150 g of 55 wt % HF aqueous solution was addedto prepare the solution D. Meanwhile, the solution E was prepared byweighing 2.66 g of KHF₂ and dissolving it in 60 g of 55 wt % HF aqueoussolution. Next, while stirring the solution D, 30.0 g of the phosphorcore obtained in Comparison Example 1 was charged in the solution D,then 1.5 g of 30 wt % H₂O₂ aqueous solution was added to reduce theeluted Mn⁴⁺ into Mn²⁺ to suppress the generation of fluoride complex[MnF₆]²⁻. In the present example, hydrogen peroxide (H₂O₂) is employedas the reducing agent, but in a method of manufacturing a phosphoraccording to the present example, the reducing agent is not limited tohydrogen peroxide and other reducing agents such as oxalic acid can alsobe employed.

[MnF₆]²⁻+2H₂O₂→Mn²⁺+4HF+2O₂↑

The [MnF₆]²⁻ in the solution D was changed to Mn²⁺ by the reaction, thenthe solution E was added to create the surface region on the phosphorparticles of Comparison Example 1 so that the concentration oftetravalent Mn is lower than that of the inner region of the phosphorparticles. The precipitation thus obtained is isolated and washed withIPA, then dried at 70° C. for 10 hours to form the fluoride phosphoraccording to Example 1.

Examples 2 to 9

The fluoride phosphors according to Examples 2 to 9 were formed in asimilar manner as in Example 1, except for changing the chargingcomposition ratio and the charged amounts which are shown in Table 2.The emission luminance characteristics of the respective fluoridephosphors obtained in this manner and the emission luminancecharacteristics after evaluating the water resistance are shown in Table3.

TABLE 3 Luminous Flux Chromaticity Relative Chromaticity RelativeMaintaining Coordinates Luminance Coordinates Luminance Rate x y (%) x y(%) (%) COMPARISON 0.677 0.313 100.0 0.678 0.312 60.4 60.4 EXAMPLE 1EXAMPLE 1 0.677 0.312 100.0 0.677 0.312 96.1 96.1 EXAMPLE 2 0.677 0.312100.0 0.677 0.312 96.9 96.3 EXAMPLE 3 0.677 0.312 99.7 0.678 0.312 94.594.7 EXAMPLE 4 0.677 0.312 97.4 0.677 0.312 97.4 100.0 EXAMPLE 5 0.6770.312 97.1 0.678 0.312 96.3 99.2 EXAMPLE 6 0.677 0.313 94.5 0.677 0.31296.3 101.9 EXAMPLE 7 0.677 0.313 93.7 0.677 0.312 95.3 101.7 EXAMPLE 80.677 0.312 92.7 0.677 0.312 92.4 99.7 EXAMPLE 9 0.677 0.312 91.3 0.6770.312 90.8 99.4

Emission Luminance Measurement Result

Prior to evaluating the water resistance, a standard testing for theemission luminance was conducted on the fluoride phosphors obtainedaccording to Examples 1 to 9 and Comparison Example 1. The relativeemission luminances of the fluoride phosphors compared to the emissionluminance of the fluoride phosphor of Comparison Example as 100% wererespectively shown in Table 3. As clearly shown in the table, therelative emission luminances are slightly decreased except for that ofthe fluoride phosphors of Examples 1 and 2. This is considered thatformation of the surface region on the phosphor particles impedes a partof the components of light to be emitted to outside.

Water Resistance Evaluation Result

The fluoride phosphors according to Examples 1 to 9 and ComparisonExample 1 will be described below. The water resistance evaluation wasconducted in such a manner that a 5 g of a phosphor was stirred in a 15g of pure water for 1 hour, then isolated, washed with IPA, and dried at70° C., for 10 hours, then, the emission luminances were compared to theemission luminances before evaluation of the water resistance. Theresults of the water resistance evaluation are shown in Table 3. Asclearly shown in the table, while the emission luminance dropped to 60.4% in the fluoride phosphor of Comparison Example 1 which does not havethe surface region described above, all the fluoride phosphors ofExamples maintain the 94% or greater luminous fluxes. Thus, validity ofthe present example was confirmed. Particularly, the fluoride phosphorsof Examples 4 to 9 respectively show higher emission luminance than thatprior to the water resistance evaluation test. This is considered that apart of the output light was initially impeded by the surface regionformed on the phosphor particles, but elusion of the surface region dueto the water resistance evaluation test allows for a reduction in suchimpeding which increases the luinance components. That is, according tothe present example, not only reducing the deterioration of the emissionluminance over time but also maintaining and improving of the emissionluminance are also confirmed.

TABLE 4 Fluorescent Calculated Material Surface Composition ElutedComposition of Analysis Amount in Fluorescent Surface Value Pure WaterMaterial Region/Inner (mol ratio) (ppm) (mol ratio) Region Mn K Mn K MnK Mn Concentration (%) COMPARISON 2.000 0.031 440 11.0 2.000 0.036 115EXAMPLE EXAMPLE 1 2.000 0.030 440 2.2 2.000 0.007 23 EXAMPLE 2 2.0000.030 420 1.3 2.000 0.004 14 EXAMPLE 3 2.000 0.029 410 1.1 2.000 0.00412 EXAMPLE 4 2.000 0.027 410 0.8 2.000 0.003 9 EXAMPLE 5 2.000 0.026 4300.7 2.000 0.002 8 EXAMPLE 6 2.000 0.026 440 0.7 2.000 0.002 7 EXAMPLE 72.000 0.023 420 1.3 2.000 0.004 14 EXAMPLE 8 2.000 0.022 430 0.6 2.0000.002 6 EXAMPLE 9 2.000 0.022 450 0.5 2.000 0.002 5

Surface Composition Analysis

Last, the fluoride phosphors respectively obtained according to Examples1 to 9, and Comparison Example 1 were charged in pure water to elude thesurfaces of the phosphor particles. Then, composition analysis isconducted and based on the composition analysis, the surfacecompositions of the fluoride phosphors were calculated. The evaluationon the composition of elution was conducted in such a manner that, 5 gof the phosphors were stirred for one hour in pure water which contains0.5% hydrogen peroxide, then the respective supernatant were taken forcomposition analysis by using ICP. The ratio of concentration of Mn inthe surface regions of the fluoride phosphors according to Examples 1 to9 were calculated with the use of the analysis values of the fluoridephosphor according to Comparison Example 1 as the inner compositions.That is, “Mn Concentration Ratio of Surface Region/Inner Region”according to Example 1 to 9 were calculated by dividing “CalculatedValue (mole ratio) of Phosphor Surface Composition” by “PhosphorComposition Analysis Values (mole ratio)” of the phosphor core accordingto Comparison Example 1. This is because “Mn Concentration of InnerRegion” of the phosphor according to each example is assumed to besubstantially the same as the Mn concentration in the phosphor coreaccording to Comparison Example 1. The results combined with thecomposition analysis results of the entire portion of the phosphorparticle with the use of ICP are shown in Table 4. As clearly shown fromthose results, the eluded amount of 11 ppm of Mn is apparently large inComparison Example 1. To the contrary, the eluded amount of Mn issuppressed to 0.5 to 2.2 ppm in each example. Thus, a low amount ofeluded Mn, in other words, a low production amount of manganese dioxideis proved, confirming that occurrence of darkening is suppressed, whichallows for less degree of degradation with time in luminance.

The fluoride phosphors according to the embodiments of the invention canbe suitably used for display devices, light source of luminaries,signaling devices, switches with lighting, various sensors, variousindicators, and so forth, and particularly suitably used for backlightof liquid crystal devices.

It is to be understood that although the present invention has beendescribed with regard to preferred embodiments thereof, various otherembodiments and variants may occur to those skilled in the art, whichare within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims. The present application is based on Applications No. 2013-140212filed in Japan on Jul. 3, 2013, and No. 2014-122,887 filed in Japan onJun. 13, 2014, the contents of which are incorporated herein byreferences.

What is claimed is:
 1. A fluoride phosphor activated with tetravalent Mnhaving a general formula shown below and being in a form of particles,to absorb visible light of a short wavelength and emit red light, eachphosphor particle having a surface region where a concentration oftetravalent Mn is lower than inner region of the phosphor particleK₂[M_(1-a)Mn⁴⁺ _(a)F₆] (wherein M is at least one selected from group-IVelements of Ti, Zr, and Hf and group IVB elements of Si, Ge, Sn, and ais 0<a<0.2).
 2. The fluoride phosphor according to claim 1, wherein theconcentration of tetravalent Mn is approximately uniform in the innerregion of each phosphor particle, and is gradually decreased toward thesurface in the surface region.
 3. The fluoride phosphor according toclaim 1, wherein the concentration of tetravalent Mn in the surfaceregion is 30% or less with respect to the concentration of tetravalentMn in the inner region.
 4. The fluoride phosphor according to claim 1,wherein an elution amount of tetravalent Mn obtained by charging thefluoride phosphor in 1 to 5 times amount of pure water relative to theamount of the phosphor to dissolve surface of the phosphor particles isin a range of 0.05 to 3 ppm.
 5. The fluoride phosphor according to claim1, wherein a luminous flux maintaining rate obtained by charging thefluoride phosphor in 1 to 5 times amount of pure water relative to theamount of the phosphor and left stand for one hour is 90% or greater. 6.The fluoride phosphor according to claim 1, wherein M is Si or Si andGe.
 7. The fluoride phosphor according to claim 1, wherein the thicknessof the surface region is about 1/20 to about 1/50 of average particlediameter of the fluoride phosphor.
 8. A light emitting device having thefluoride phosphor according to claim 1, comprising a light source toemit light of visible light at a shorter wavelength side, and thefluoride phosphor to absorb the light and emit a red light.
 9. The lightemitting device according to claim 8, wherein the light emitting deviceincludes, in addition to the fluoride phosphor, at least one type ofphosphor selected from (Ca, Sr, Ba)₂SiO₄:Eu, (Y,Gd)₃(Ga,Al)₅O₁₂:Ce,(Si,Al)₆(O,N)₈:Eu, SrGa₂S₄:Eu, (Ca,Sr)₂Si₅N₈:Eu, CaAlSiN₃:Eu, and(Ca,Sr)AlSiN₃:Eu.
 10. The light emitting device according to claim 8,wherein the light emitting device is used as backlight of liquidcrystal.
 11. A method of manufacturing a fluoride phosphor activatedwith tetravalent Mn to absorb visible light of a short wavelength andemit red light, having a general formula K₂[M_(1-a)Mn⁴⁺ _(a)F₆] (whereinM is at least one selected from group-IV elements of Ti, Zr, and Hf andgroup IVB elements of Si, Ge, Sn, and a is 0<a<0.2); the methodcomprising: forming phosphor cores whose composition is represented bythe general formula including mixing a solution which contains at leastMn and F, a solution which contains at least K and F, and a solutionwhich contains at least Si and F; and mixing the phosphor cores and asolution containing a reducing agent to form a surface region on each ofthe phosphor cores so that a concentration of tetravalent Mn on thesurface region is lower than in inner region of the phosphor particle.12. The method of manufacturing a fluoride phosphor according to claim11, wherein the reducing agent is hydrogen peroxide.
 13. The method ofmanufacturing a fluoride phosphor according to claim 11; wherein thesurface region is formed by mixing the phosphor cores, a solution whichcontains at least Si and F, a solution which contains at least K and F,and a solution which contains a reducing agent.
 14. The method ofmanufacturing a fluoride phosphor according to claim 13; wherein thesurface region is formed by charging the phosphor cores in a solutionwhich contains at least Si and F, and adding a solution which contains areducing agent and a solution which contains at least K and F.